The present invention relates generally to a system for prolonging the life of a signal transfer line disposed at a subsurface location, and particularly to a system for protecting a signal transfer line, such as those containing electric cable and/or optic fiber, in a downhole, wellbore environment.
A variety of tools are used at subsurface locations from which or to which a variety of output signals or control signals are sent. For example, many subterranean wells are equipped with tools or instruments that utilize electric and/or optical signals, e.g. pressure and temperature gauges, flow meters, flow control valves, and other tools. (In general, tools are any device or devices deployed downhole which utilize electric or optical signals.) Some tools, for example, may be controlled from the surface by an electric cable or optical fiber. Similarly, some of the devices are designed to output a signal that is transmitted to the surface via the electric cable or optical fiber.
The signal transmission line, e.g. electric cable or optical fiber, is encased in a tube, such as a one quarter inch stainless steel tube. The connection between the signal transmission line and the tool is accomplished in an atmospheric chamber via a connector. Typically, a metal seal is used to prevent the flow of wellbore fluid into the tube at the connector. This seal is obtained by compressing, for example, a stainless steel ferrule over the tube to form a conventional metal seal.
However, the hostile conditions of the wellbore environment render the connection prone to leakage. Because the inside of the connector and tube may stay at atmospheric pressure while the outside pressure can reach 15,000 PSI at high temperature, any leak results in the flow of wellbore fluid into the tube. The inflow of fluid invades the internal connector chamber and interior of the tube, resulting in a failure due to short circuiting of the electric wires or poor light transmission through the optic fibers. This, of course, effectively terminates the usefulness of the downhole tool.
Additionally, the signal transfer lines often extend through the protective tube over substantial distances, e.g. to substantial depths. If not supported, the weight of the signal transfer lines creates substantial tension in the lines that can result in damaged wires/fibers. Even if the signal transfer lines can withstand the tension, any cutting of the wires/fibers results in severe retraction of the lines into the tube. For example, when a technician cuts the lines to repair a damaged cable or to cross a tubing hanger, packer, annulus safety valve, another tool etc., the retraction occurs.
A common solution is to add a filler in the annulus between the interior surface of the tube and the wires and/or fibers. The filler may comprise a foam rubber designed to expand with temperature to fill the gap between the signal transfer lines and the interior surface of the tube. However, such a filler does not alleviate the problem of substantially reduced interior pressure relative to the exterior pressure that can result in the inflow of deleterious wellbore fluids.
It would be advantageous to have a system for preventing the inflow of wellbore fluids into contact with signal transmission lines disposed within a protective tube.
The present invention provides a technique for preventing damage to signal transmission lines, such as electric wires and optical fibers, utilized in a high pressure, subsurface environment. The system utilizes signal transmission lines deployed in the interior of a tube, such as a stainless steel tube, extending to a subsurface location, such as a downhole location within a wellbore.
The signal transmission lines are designed for connection to a tool, while the tube is attached to the tool by a connector. The connector typically also has an interior chamber. The interior chamber of the connector is filled with a pressurized fluid, such as a liquid, and pressurized until the internal pressure is greater than the external pressure acting on the connector. Thus, if leaks form about the connector, the flow of fluid is from the connector to the wellbore rather than from the wellbore into the connector.
In at least one embodiment, the high pressure fluid is supplied to the connector chamber via a fluid communication path within the interior of the tube. Preferably, the tube interior also is maintained at a higher pressure than the surrounding environmental fluid at any given location along the tube.